For general background regarding photocathodes, reference is made to the following patents: U.S. Pat. No. 4,999,211, issued Mar. 12, 1991, in the name of Daniel D. Duggan, entitled "Apparatus and Method for Making a Photocathode" ("Duggan '211"); U.S. Pat. No. 5,114,373, issued May 19, 1991, in the name of Robert Peckman, entitled "Method for Optimizing Photocathode Photoresponse" ("Peckman '373"); and U.S. Pat. No. 5,298,831, issued Mar. 29, 1994, in the name of Avrham Amith, entitled "Method for Making Photocathodes for Image Intensifier Tubes." Each of these patents are assigned to ITT Corporation, the assignee herein, and they are incorporated herein by reference. As noted above, photocathodes are useful in image intensifier devices, photomultiplier tubes, photon counter, and light-controlled electron sources, as in lithography and electron accelerators. Image intensifier tubes have industrial and military applications, such as for enhancing the night vision of aviators, photographing astronomical bodies, and providing night vision to people who suffer from retinitis pigmentosa (night blindness).
A photocathode typically has a surface for receiving light and a photoemissive layer oppositely disposed to the light-receiving surface; light may be emitted from the photoemissive layer to an electron amplifier (or microchannel plate) and to an anode. The photoemissive layer of the photocathode is generally comprised of several layers of material. An anti-reflective layer is disposed on the surface facing the microchannel plate, which can be fabricated with silicon nitride (Si.sub.3 N.sub.4). This is followed by a window layer and an active layer, both of which may be composed of gallium aluminum arsenide (GaAlAs), and gallium arsenide (GaAs), respectively. Lastly, it has been found that an activation chemical, such as cesium oxide, cesium fluoride, or barium fluoride, applied to the surface of the gallium arsenide is effective in increasing the quantity of electrons emitted from the photoemissive wafer.
For the photocathode to generate a flow of electrons, the photons of light directed at the photocathode must be captured in the active layer of the photoemissive wafer. This capturing of photons causes a release of electrons from the photoemissive wafer. The electrons must be released and proceed to the microchannel plate before being re-absorbed in the wafer. The activation layer serves to reduce the attractive force of the electrons so that a large portion of them escape the wafer, resulting in a large photo-response (PR). The amount of energy needed for an electron to leave the surface of the photocathode is called the work function. Photocathodes that are activated with an activation chemical can actually have a negative work function, so that electrons are repelled from the surface of the photocathode.
The activation layer is typically deposited on the photocathode surface by a vacuum deposition process. The current technology is to activate (apply an activation chemical to lower the work function and increase the PR), one photocathode at a time. For example, to deposit a cesium oxide activation layer, a photocathode is disposed in a vacuum chamber, and cesium gas is directed into the chamber and deposited on the photocathode, followed by an influx of oxygen gas. As the cesium and oxygen are deposited, electrons are released from the gallium arsenide layer. The rate at which the electrons are released, or the photoresponse (PR), is monitored during this process, as the photocathode is electrically connected to a power source and a PR meter. The stoichiometry of the activation chemicals is then adjusted during the application process based on the PR, as displayed by the PR meter.
Under current technology, the photocathodes have been activated individually. In fact, it had been believed that individual activation was necessary because of complexities involved in simultaneously monitoring the PR while adjusting the flow of activation chemicals. Imperfections in the activation surface are attractive sites to the electrons and regions immediately surrounding them have no PR, such that maintaining a properly uniform coating of activation chemicals is significant in improving the overall effectiveness of the photocathode. It had been thought that the differences in PR of the photocathodes, coupled with difficulties inherent in adjusting the stoichiometry of the activation chemicals, required an individualized application process.
To illustrate, in the activation process, cesium is introduced into the vacuum chamber by chemically reacting a cesium compound, such as cesium chromate, to release a diffused molecular vapor of cesium inside the vacuum chamber. The cesium on the cathode surface produces an increase in photo response which peaks after about 5 minutes. Then oxygen is introduced to co-deposit with the cesium to produce a cesium oxide. The flux of oxygen is controlled with a valve to maintain the correct stoichiometry. The stoichiometry is monitored by adjusting the oxygen flux so as to maximize the PR, while the cesium flux remains constant. And thus, it had been thought that an individualized process was necessary to make proper adjustments to the stoichiometry of the activation chemicals.
Shortcomings exist with regard to an individualized process. In particular, greater process time is necessarily consumed when activation layers are applied individually to one photocathode at a time.
Therefore, it is an object of the instant invention to provide a method of applying an activation layer to the photoemissive surface of the photocathode that may be used to simultaneously apply such activation layers to a plurality of photocathodes.